The present invention relates to an improved process for preparing yeast. More particularly, it relates to a process which produces yeast in improved yields and of improved quality.
Commercial yeast production typically entails propagation in a plurality of stages. Generally, yeast are innoculated into a presterilized nutrient medium usually contained in a shaker flask. In the flask, growth of the yeast is encouraged by various means such as controlling the temperature and shaking the flask to effect aeration. The yeast are removed from this flask and innoculated into another flask containing a larger volume of nutrient medium for continued growth. These initial stages may conveniently be referred to as flask or culture development stages.
From the culture development stages, the yeast may be innoculated into a vessel having an air source and means of agitation. These steps may be repeated once or twice using greater amounts of nutrient medium and larger vessels. Because the amount of air used in these stages is generally restricted, these stages are commonly referred to as slightly aerobic stages. Yeast from these stages are then transferred into larger fermentors where vigorous growth conditions are maintained, including the use of large volumes of air. These stages may be referred to as highly aerobic, or commercial, stages since the yeast from these stages are harvested and processed for bakery or home use, typically in compressed or active dry form.
For propagation in the highly aerobic or commercial stages, it is necessary to prepare large quantities of a yeast culture medium which is substantially free of microorganisms. This has been accomplished in the past by sterilizing the medium such as final molasses, by heat treatment. To reduce the microorganism count to a level effective to produce yeast suitable for food use, large amounts of energy, as well as means for generating and transferring heat to the process, were required. Typically, the heat was generated in oil or gas-fired boilers and transferred to the process as steam which could be injected live or transferred by means of heat exchangers. Thus, this sterilization step entailed sizable capital and operational costs. It would be desirable to reduce these costs.
Typically, the culture medium would contain molasses. Molasses is the thick liquid which is left after sucrose has been removed from the mother liquor in sugar manufacture from either beets or cane. Molasses does not have an absolutely fixed composition because of the many variations in commercial sugar production and the various stages in the process at which it may be withdrawn. Typically, however, a product known as "final molasses" contains about 20% sucrose, 20% reducing sugars, 10% ash, 20% non-sugar organic materials, and 20% water. This product is essentially the syrup which remains when it is no longer commercially practical to remove further sucrose. This product, also known as "black strap molasses" is typically utilized to produce yeast, vinegar, and various organic chemicals, such as alcohols, through fermentation.
In U.S. Pat. No. 4,101,338, to Rapaport et al, there is disclosed a process for fractionating carbohydrate-containing materials, such as molasses, which enables further extraction of sugars. According to that procedure, several fractions are obtained by contacting the material with an ion exchange resin. In one embodiment, ultrafiltration is employed to remove higher molecular weight color bodies from the molasses prior to fractionation. The process does not, however, utilize the decolorized molasses for production of yeast nor other known uses, but fractionates the molasses into a number of new products, each having independent utility and economic value.
In recent years, ultrafiltration membranes have been employed in a number of unit operations to remove bacteria, separate polysaccharides based on molecular weight, remove ash, and a number of other procedures. For example, U.S. Pat. No. 3,228,877 to Mahon, U.S. Pat. No. 3,419,144 to Huntington, U.S. Pat. No. 3,974,068 to Ebner et al and U.S. Pat. No. 3,982,024 to Oneto, all employ membranes to remove microorganisms from solutions. In addition to the discussion by Rapaport et al that color may be removed from carbohydrate-containing solutions, similar disclosures are found in U.S. Pat. No. 4,115,147 to Shimizu et al wherein a nutritive sugar is produced by a non-centrifugal process employing ultrafiltration, and in U.S. Pat. No. 4,211,577 to Wallin which teaches the extraction of anthocyanin colors from materials such as grape juice. In addition to color removal, Shimizu further teaches the removal of ash and high molecular weight components. Also relating to the removal of ash from raw sugar juice is U.S. Pat. No. 3,799,806 to Madsen. Among the many further disclosures which relate to the removal of high molecular weight components, are those of Rapaport and Shimizu mentioned above, and also U.S. Pat. No. 3,668,007 to Egger et al, U.S. Pat. No. 3,756,853 to Meyer, U.S. Pat. No. 3,832,285 to Kurimoto, and U.S. Pat. No. 4,069,103 to Muller. However, to the best of my knowledge, current commercial yeast production does not employ ultrafiltration to pretreat the molasses employed as the yeast culture medium. Moreover, the prior art does not anywhere suggest that yields of yeast based on the quantity of molasses employed, nor the leavening activity of the resulting yeast based on the effectiveness per weight of product yeast, could be improved by a process employing ultrafiltration.